The major chemical pathways for drug metabolism are hydrolysis, conjugation, oxidation and reduction. Of these four areas the last - metabolism by reduction - has been, in some respects, the least well appreciated in its scope and importance. An important area of reductive metabolism is the reductive activation of antitumor antibiotics. These antibiotics are redox-active, and require metabolic activation, by reduction, following their administration in order for the expression of their biological activities. We have initiated a program to examine the chemical mechanism of this process for two quinone antibiotics, the anthracyclines and the mitomycins. The questions this program addresses are the following. What are the enzymes involved in the reduction process? What variables determine how the rates, yields, and products balance between competing reactions? What is the efficiency of the secondary reactions between the activated antibiotics and possible biological targets? The answers to these questions are being sought through the mechanistic study of chemically well defined systems. These consist of an appropriate reducing agent, an enzyme catalyst to mediate electron flow to the catalyst, and an appropriate target molecule. The kinetics of the reductive activation process are determined and the products of the reaction are identified. Within a defined protocol, the variables that are evaluated include the relative efficiency of the enzymes, the effect of antibiotic structure, and the relative reactivities of the activated quinones with potential nucleophiles. From this information, a more complete characterization of the chemistry of the mitomycin and anthracycline antibiotics is obtained. This will allow the assessment of the chemical features of these antibitics that influence their biological activities of acute toxicity, mutagenicity and tumor cytotoxicity.